This invention relates to a time division multiplexer capable of repeating data from one time division multiplexer to another.
A time division multiplexer is an apparatus which combines (multiplexes) and separates (demultiplexes) high speed digital data being transferred between numerous data terminals and a digital transmission line. Many kinds of data terminals may be involved, such as a PBX or a host computer.
This invention concerns communication systems composed of three or more time division multiplexers. To connect each time division multiplexer to every other one by a digital transmission line is not economical because of the number (and, often, the length) of digital transmission lines involved. Generally, it requires n(n-1)/2 lines to connect n time division multiplexers in this manner. Consequently, it has been suggested that some time division multiplexers be connected to others only via a third multiplexer which works as a repeater, as shown in FIG. 1a. In FIG. 1a, six time division multiplexers A to F are connected by five digital communication lines a to e. Time division multiplexer A and D have a repeating function, performed by the multiplexing and repeating unit. In this system time division multiplexers B and F are interconnected only through the multiplexing and repeating unit of time division multiplexer A; similarly, multiplexers C and E are interconnected only through the multiplexing and repeating units of time division multiplexers A and D. Economical communication systems are therefore built using the repeating function.
To realize this repeating function, the apparatus shown in FIG. 1b was considered by the inventors. The time division multiplexer includes four multiplexing and repeating units AA to AD. Each multiplexing and repeating unit is connected to a respective digital transmission line a to d and to a respective bus ba to bd to which a plurality of data terminals DT are commonly connected. Multiplexing and repeating units AA to AD are interconnected by repeating buses bab to bed. Multiplexing and repeating unit AA demultiplexes the data train received from the other time division multiplexers and delivers it to their respective data terminals or to another multiplexing and repeating unit. Data delivered to unit AA is temporarily stored in its memory before being multiplexed with other data from the data terminal. The multiplexed data train is sent to other time division multiplexers.
In such a system, each multiplexing and repeating unit has to be provided with memory and with transfer switches between all of the lines and a plurality of repeating buses. Therefore, the electrical structure of each multiplexing and repeating unit is complicated. Also, it requires a large-scale change in all existing multiplexing and repeating units whenever another unit is to be added. For example, to go from three to four, first three new repeating buses are required to interconnect four units. And each multiplexing and repeating unit has to be changed as to the transfer switch functions. So, the system like that shown in FIG. 1b has less flexibility.